It is known that a single microwave radiation source can be used to generate a plasma by subjecting a gas to a sufficient amount of microwave radiation. A single microwave energy source can be damaged, however, when one source directs microwave energy into a plasma chamber that reflects the energy back toward the same source. This can be especially problematic when there is no strong microwave absorber in the cavity, such as when a plasma is not yet formed. Also, multiple microwave energy sources are particularly susceptible to damage when combined to ignite or sustain a plasma. For example, a first source can be damaged when it directs microwave energy into a chamber because that energy could be directed into another simultaneously connected radiation source.
It is also known that plasma ignition is usually easier at gas pressures substantially less than atmospheric pressure. However, vacuum equipment, which is required to lower the gas pressure, can be expensive, as well as slow and energy-consuming. Moreover, the use of such equipment can limit manufacturing flexibility.